The invention relates to a speed control for controlling the speed of an electric induction motor by modulating the driving voltage, particularly the amplitude and frequency of this voltage.
Such a speed control is known from United States patent specification U.S. Pat. No. 4,327,315. Said patent specification describes an induction motor in which an LF motor voltage (a voltage of a comparatively low frequency of approximately 1-100 Hz) is obtained by means of an HF pulse width control (pulse width control with a comparatively high frequency of approximately 1-100 kHz). The speed control is then based on the so-called V/f principle. This means that the motor voltage is raised proportionally as the speed (f) increases. In this way, the amount of flux in the motor coil, expressed in V.s (volts second), remains constant and the maximum motor torque remains constant.
In an electric motor the driving voltage builds up a rotating field in the stator, which is followed by the rotor. By modulating the driving voltage the speed of the motor can be influenced. In general, it holds that the speed will increase by increasing the driving voltage and/or by increasing the frequency of the driving voltage. In a no-load condition the rotor will run in synchronism with the rotating field. When loaded the motor rotates at a speed which can be up to approximately 80% as low as the rotating field frequency. An induction motor develops a motor torque only by virtue of a difference in speed of the rotor and the frequency of the rotating field built up between the poles of the stator (the so-called slip). Hereinafter, frequency is to be understood to mean, unless otherwise stated, the frequency of the rotating field built up in the stator, i.e. the frequency of the driving voltage.
The speed assumed under load is referred to as the operating point. When the motor torque is constant this operating point will be shifted toward increasingly lower speeds as the load increases.
Induction motors for electrical appliances are popular in the industry because they can be manufactured comparatively cheaply. However, the possibilities of using motors of this type are limited for uses at lower speeds, particularly in the cases where the load torque at low speeds is comparatively small. As a result of the difference between the motor speed and the rotating field frequency the rotor is alternately accelerated and decelerated in one cycle of the rotating field. The frequency of this pulsation is twice the frequency of the rotating field. As a result of this pulsating torque the motor runs less steadily and produces more noise at lower speeds.
Such a situation occurs, for example, in the case of a fan or a pump. With such appliances the load torque at low speeds is substantially smaller than the motor torque, as a result of which the motor begins to xe2x80x9cpoundxe2x80x9d and no longer runs steadily owing to the pulsating torque and saturation effects. Known speed controls are unsatisfactory in particular in the case that an electric induction motor is used in the fan of an air cleaner. Such an air cleaner feeds the air through a number of cleaning filters at a comparatively low velocity. The sound effects produced by the air stream are then comparatively small. Nevertheless, the noise level produced by the air cleaner owing to the presence of the fan is fairly high. This effect is experienced as annoying.
It is an object of the invention to eliminate said drawbacks and to provide a speed control which makes the electric induction motor run silently and steadily at low speeds. It is another object of the invention to provide an electric motor having such a speed control, which can be manufactured simply and cheaply, particularly for use in a fan or pump. In a speed control of the type defined in the opening paragraph this object is achieved in that the speed control is adapted to control the amplitude V of the driving voltage in accordance with the formula V=b.fx when the frequency f varies, in which formula x has a value greater than 1 and smaller than 3 and b is a constant.
In a preferred embodiment the value of x is greater than 1.5 and smaller than 2.5, and is preferably 2. This speed control has the advantage that at low speeds the maximum motor torque is much smaller than with the known V/f control, as a result of which the motor runs better and more silently. Since the motor torque is always optimized for the load saturation effects at low speeds are less audible. Harmonic distortion in the rotating field in the stator is less likely to occur, as a result of which the motor is more silent. When an electric motor is used as a fan or pump the load torque is theoretically a square-law function of the speed. This means that the load torque at lower speeds is much smaller than the motor torque of a motor which is operated with a motor voltage proportional to the speed, as is the case in the said United States patent specification. The speed control then controls the amplitude of the driving voltage proportionally to the square of the frequency, as a result of which the value x is 2.
When the speed control is used in an electric motor of an air cleaner this cleaner can operate very silently at lower speeds.
The invention can be used with a multi-phase driving voltage, for example a three-phase driving voltage. However, this type of driving voltage is not customary with motors for domestic uses, particularly with motors for an air-cleaner fan. In addition, the effect is less pronounced in the case of multi-phase driving voltages because at lower speeds these motors operate more steadily anyway. In a preferred embodiment, however, the driving voltage for the motor is a single-phase voltage. Motors with a single-phase driving voltage are very suitable for domestic appliances because they can be manufactured simply and cheaply. In addition, the required power is comparatively low for domestic uses. The use of the speed control in accordance with the invention offers advantages particularly with motors of this type because hitherto these motors were less suitable for low speed drives.
Preferably, the driving voltage is approximately sinusoidal. The rotating field which is built up by said voltage then has a minimal distortion, as a result of which annoying vibration and noise effects are minimal.
For realizing a speed control in accordance with the invention various circuits are conceivable, both in analog and in digital versions. In a preferred embodiment the driving voltage is modulated by pulse width control. This form of modulation makes it possible to modulate high power levels with the aid of power transistors in a comparatively simple manner.
In a further preferred embodiment the pulse width is modulated by the output voltage of a digital signal processor (DSP). A cheap and functional embodiment is then a DSP which supplies an output voltage having at least six discrete voltage levels which approximate a sinewave voltage.
The invention further relates to an electric induction motor having a speed control in accordance with one of the afore-mentioned embodiments.
Although different types of electric induction motors are conceivable in which the invention can be used, the motor preferably has a squirrel-cage armature. In this type of motor there is no electrical connection with the stationary part of the motor, as a result of which the motor is more silent and has a longer lifetime.
In a further preferred embodiment the motor is of the shaded-pole type. In this type a part of the stator is screened, as a result of which an asymmetry in the rotating field is produced. This enables the motor to be started from standstill, without the additional cost of an auxiliary winding or capacitor. The efficiency of a shaded-pole electric motor is comparatively low. However, this need not be a drawback in the case of the comparatively low power ratings required for a fan or air cleaner.
The invention also relates to a fan, a pump or an air cleaner having an electric induction motor in one of the embodiments described herein.